Ten zeolite-bearing rocks rich in HEU-type zeolites from Evros District and Samos island were studied. The samples were heated at 450°C for 8 h and the reductions in the intensity and the peak area of the (020) reflection was measured. Electron microscope and thermal analyses were used, as well. The results of the above methods gave an explicit picture for the HEU-type phases that exist in each sample and they led to their regrouping in three different groups: group I where the main phase of determined HEU-type zeolite was clinoptilolite, group II, presenting mediate characteristics between groups I and III, where the contained main phase of HEU-type zeolite is intermediate heulandite, and group III, where heulandite is the main HEU-type zeolite phase. The combined study of the XRD results in initial and thermally treated samples, compared with the SEM-EDS results and the thermal study of the mass recovery during the cooling stage of the thermal analysis constitute a useful tool for the classification of the HEU-type zeolites in their different phases.